As the hollow cylindrical workpiece that becomes an object to be heat-treated, for example, there is a bushing being one of components for crawler of a construction vehicle. However, it does not mean that the hollow cylindrical workpiece is limited to the bushing for crawler merely.
A bushing for crawler is a component which transmits power while an inner circumferential surface is in contact with a pin and an outer circumferential surface is in contact with a sprocket. Therefore, strength, toughness, wear resistance property (abrasion resistance property) and so on are required to a bushing for crawler.
In order to satisfy requirements such as the strength, the toughness, the wear resistance property and so on of a bushing, it is known there is a manufacturing method (or a heat treatment method) including a step of performing outer circumference quenching of the bushing and a step of performing inner circumference quenching of the bushing. Here, as a cooling method in the step of performing inner circumference quenching of the bushing, conventionally, a technique (for example, Patent Literature 1) of cooling by injecting a cooling liquid to an outer circumferential surface of the bushing has been carried out widely.
Recently, it is required to improve the wear resistance property by increasing a depth of a hardened layer on the outer circumferential surface side while maintaining a hardened layer depth on the inner circumferential surface side of the bushing in a fixed range. In order to fulfill such the requirements, it is insufficient to carry out cooling step by injecting a cooling liquid to a bushing outer circumferential surface merely, and also, it is necessary to cool a bushing inner circumferential surface side.
In the prior art, as a method of cooling an inner circumferential surface side of a bushing, there is a method in which a cooling jacket is inserting into the bushing and the cooling liquid is injected from the cooling jacket (see Patent Literature 2).
In addition, there is a cooling method in which cooling the inner circumferential surface side of the bushing is carried out by attaching the cooling jacket to a leading end part of a heating coil which has been inserted into the bushing inner area and injecting the cooling liquid from a concerned the cooling jacket.
Further, a technology for cooling the inner circumferential surface side of the cylindrical component is proposed, in which technology a liquid flow control member is set in an inside area of a cylindrical component and a cooling liquid is injected from a cooling liquid injecting device having been arranged at an outside area of the cylindrical component (see Patent Literature 3).
However, in the prior art in which the cooling jacket is inserted into the bushing and the cooling liquid is injected from the inserted cooling jacket, it is necessary to wait a completion of process for replacing a bushing having been carried out a heat treatment by a bushing to be subjected to heat treatment, until the cooling jacket being inserted into the bushing is returned to an original position thereof (a position at which the cooling jacket is out of the inner area of the bushing) even after quenching of the bushing has been completed by injecting the cooling liquid to the bushing inner circumferential surface side. Therefore, there is a demerit for decreasing the effectiveness for manufacturing bushings.
In addition, since the cooling jacket has to be moved frequently so as to reciprocate between the position (the original position) at which the cooling jacket is out of (detached from) the bushing inner area and the position (a position after inserted) at which the cooling jacket is inserted into the bushing inner area, a so-called “off-center” is generated easily in the cooling jacket and there is an adverse effect of uniformity in quenching quality.
In addition, in the prior art in which a bushing inner circumferential surface side is cooled with the cooling liquid having been injected from the cooling jacket attached to the leading end part of the cooling coil being inserted into an inner area of the bushing, it is necessary to form a path for the cooling liquid in an area along with a center line of the cooling coil. Therefore, there is a case that the cooling jacket cannot be inserted into a workpiece being small in inner diameter.
In addition, since the diameter of an injection orifice in the cooling jacket is small, impurities in the cooling liquid will clog the injection orifice easily. In a case that such clogging is generated, the cooling liquid cannot be injected and a quenched structure becomes non-uniform, and therefore, there is a problem that a quality abnormality of the bushing as a heat-treated product is generated.
Since it is also necessary to insert the liquid flow control member and to form the path of the cooling liquid in the cylindrical component in the prior art in which the liquid flow control member is set in an inside are of the cylindrical component and the cooling liquid is injected from the cooling liquid injecting device provided in an outside area of the cylindrical component so as to cool the inner circumferential surface side of the cylindrical component, there is a problem that it is impossible to apply such the prior art for a workpiece being small in inner diameter.